The internal grinding of a long bore with an oil or similar axial or helical groove within close bore roundness tolerance has been fraught with difficulties in the past. Typically, the roundness of the bore is difficult to maintain within a given tolerance within the vicinity of the interruption, i.e., oil or other groove, in the bore wall.
It is thought that the interruption in the bore wall contributes to out-of-tolerance bore roundness by allowing loss or collapse of the hydrodynamic coolant pressure film in the vicinity of the groove. That is, the groove provides a ready escape path for the coolant film. Prior art workers have attempted to deal with this problem by grinding without coolant altogether or by discontinuing liquid coolant flow after rough grinding and before finish grinding or after rough and finish grinding but before the final spark-out or dwell phase of grinding during which the grinding wheel is not fed into the workpart bore wall but rather is allowed simply to remain in contact with the bore wall until grinding force decreases to the so-called threshold level below which no further grinding occurs. However, it has been very difficult to determine and establish the right conditions for effecting sufficient dry grinding to round-up the bore without damaging the grinding wheel so as not to compromise bore straightness and surface finish. Attempts to reduce, rather than completely shut off, flow of the liquid coolant stream have also failed as a result of the relatively small quantity of coolant involved and the great difficulty in controlling or limiting its distribution uniformly along the length of the bore.